Air Pressure Checks
Air Pressure Checks AOD
A NO DRIVE condition can exist even with the correct transmission fluid pressure because of inoperative clutches or bands. On automatic transmissions, an erratic shift can be located through a series of checks by substituting air pressure for fluid pressure to determine the location of the malfunction.
When the selector lever is at 2 (second) a NO DRIVE condition may be caused by an inoperative forward clutch. A NO DRIVE condition at D (drive) may be caused by an inoperative forward clutch or one-way clutch. When there is no drive in 1 (low) the difficulty could be caused by improper functioning of the forward clutch or simultaneous malfunction of the low-reverse band and the one-way clutch. Failure to drive in R (Reverse) could be caused by a malfunction of the reverse-high clutch or low-reverse band or clutch.
When you have a slip problem but don't know whether it is in the valve body or in the hydraulic system beyond the valve body, the air pressure tests can be very valuable.
Fig. 15
Fig. 16
To make the air pressure checks, loosen the oil pan bolts and lower one edge to drain the transmission fluid. Remove the oil pan and the control valve body assembly. The inoperative clutches or bands can be located by introducing air pressure into the various transmission case passages (Figs. 15 and 16).
Forward Clutch
Apply air pressure to the transmission case forward clutch passages (Figs. 15 and 16). A dull thud can be heard when the clutch piston is applied. If no noise is heard, place the finger tips on the input shell and again apply air pressure to the forward or front clutch passage. Movement of the piston can be felt as the clutch is applied.
Governor
Apply air pressure to the control pressure to governor passage (Figs. 15 and 16) and listen for a sharp clicking or whistling noise. The noise indicates governor valve movement.
Reverse-High Clutch
Apply air pressure to the reverse-high clutch (Figs. 15 and 16). A dull thud indicates that the reverse-high or rear clutch piston has moved to the applied position. If no noise is heard, place the finger tips on the clutch drum and again apply air pressure to detect movement of the piston.
Intermediate Servo
Hold the air nozzle in the front servo apply tube or the intermediate servo apply passages (Figs. 15 and 16). Operation of the servo is indicated by a tightening of the front or intermediate band around the drum on C5 and C6 transmissions. Continue to apply air pressure to the servo apply tube or passage, and introduce air pressure into the front release tube or the intermediate servo release passage. The front or intermediate servo should release the band against the apply pressure.
C5
AOD
Low-Reverse Servo--C5 Only
Apply air pressure to the low-reverse servo (Figs. 15 and 16). On C5 and AOD, the low-reverse band should tighten around the drum if the servo is operating properly.
If the servos do not operate, disassemble, clean, and inspect them to locate the source of the trouble.
If air pressure applied to either of the clutch passages fails to operate a clutch or operates both clutches at
once, remove, and with air pressure, check the fluid passages in the case and front pump to detect obstructions.
Low-Reverse Clutch C6 Only
Apply air pressure to the low-reverse clutch apply passage (Fig. 16). A dull thud should be heard if the clutch is operating properly. If the passages are clear, remove the clutch assemblies, and clean and inspect the malfunctioning clutch to locate the trouble.